1. Technical Field
This disclosure relates to removing contaminants from a gas.
2. Description of Related Art
There are many instances from industrial processes to counterterrorism where a gas includes gas phase contaminants that need to be removed. Commonly used methods to remove such contaminants include adsorption beds.
For example, landfill gas (LFG) and biogas are important renewable fuels as they contain more than 50% methane. LFG is a product of the biodegradation of waste materials deposited in landfills. Biogas may be produced from a variety of sources including the biodegradation in digesters of sludge from waste water treatment plants (WWTP). Typically, LFG and biogas include numerous trace contaminants, which are often referred to as non-methane organic compounds (NMOC). In some instances, the NMOC in LFG and biogas may contain halogens and sulfur. When these fuels are combusted to generate energy (e.g., in an engine or a turbine), the halogens and sulfur may produce acids that corrode the equipment and contribute to pollution (e.g., acid rain). Another class of NMOC is siloxanes, which contain in their structure silicon. When combusted, siloxanes may produce silica microparticles. In the energy producing equipment, the silica microparticles may coat the internal surfaces of the equipment. When the microparticles escape in the flue-gas, the particulate emissions may be both an environmental and health hazard.
One method for removing NMOC from LFG or biogas is to dehumidify the gas, remove the hydrogen sulfide, and then use active carbon (AC) beds to remove the NMOC. However, this method may not be particularly selective to NMOC that include halogens, sulfur, or silicon. Rather, the method typically removes most NMOC including aromatic compounds that may be harmless in energy producing equipment. Therefore, the AC beds may saturate more quickly, which gives rise to frequent regeneration cycles of the beds to mitigate breakthrough of the NMOC that are harmful in the energy producing equipment. Additionally, because the AC beds rely primarily on adsorption, during regeneration the NMOC are desorbed. The desorbed NMOC may then be flared, which produces air pollution and wastes fuel in the flaring process.
In another example, chemical warfare agents (CWA) may be dispersed in an air supply. As with the removal of NMOC from LFG, some methods for CWA removal may use adsorption with AC beds. In this example, because CWA are typically potent at low concentrations, the air may need to be purified to very low levels of CWA. However, AC beds rely on physisorption for removal of gas phase contaminants, which is highly concentration-dependent. Therefore, in some instances, the necessary level of air purity may be difficult to attain. Also, other components in the air like volatile organic compounds (VOC) and water may also adsorb in the AC beds, which reduces the capacity for CWA adsorption. Additionally, because the gas phase contaminant being removed is a CWA, the AC beds must be disposed of (which presents exposure issues) and be replaced, which makes the methods and systems discontinuous and more labor and equipment intensive to operate.